Converting metric prints to imperial

[GKNIPP]

I noticed in one of the threads in this section that there is a metric conversion table posted somewhere on HMEM, however, was unable to locate it. I would be able to easily accomplish this task via long hand arithmatic but felt the conversion chart on HMEM would be more expedient.

I would appreciate anyones help in finding this link as I have obtained a set of BONZER hit-n-miss engine plans drawn in metric dimensions and need to convert them to imperial.

Thank you in advance for your help.

Greg

 

[GWRdriver]

Greg,
The easiest way for me to become handy with Metric was to memorize the number .03937. Some folks do it the other way around and memorize 25.4.

1.000" = 25.4mm
1.000" ÷ 25.4 = .03937 = 1mm

Just multiply by .03937, or or divide using 25.4, and you will have the Imperial inch equivalent. A small calculator will be a help.

For example . . . if a dimension is 77.25mm, multiply that by .03937 and the result is 3.0413". Divide by 25.4 and the result is 3.0413".

 

[basement_guy]

I converted some plans from inch to metric and found this list very useful.
http://mdmetric.com/tech/cvtcht.htm

 

[Woodster]

A word of caution!! To divide by 25.4 is EXACT, to multiply by .03937 is APPROXIMATE.

 

[Ken I]

A further word of caution in that dividing by 25.4 is going to give you some really odd numbers - you then need to round them up or down to the nearest fractional or decimal equivalent (plus the same for matching parts) as well as the closest threads etc.

Otherwise just execute it in metric.

Since I work in metric and all my taps, reamers etc are metric my problem is usually the other way around - typically I redraw the imperial plans into the nearest metric.

Metricating / Imperialising is not as simple as just multiplying by some constant.

There are some other threads on this site giving some rules of thumb guidance for this.

Ken

 

[trumpy81]

I noticed in one of the threads in this section that there is a metric conversion table posted somewhere on HMEM, however, was unable to locate it. I would be able to easily accomplish this task via long hand arithmatic but felt the conversion chart on HMEM would be more expedient.

I would appreciate anyones help in finding this link as I have obtained a set of BONZER hit-n-miss engine plans drawn in metric dimensions and need to convert them to imperial.

Thank you in advance for your help.

Greg

Do you use CAD or 3D modelling at all?

If you 3D model you can make the conversion in any way you wish. Rounding up or down as you go and you have the benefit of checking clearances and looking for obstructions along the way too.

It would not be the first time I have discovered some anomalies between the model and the drawings which would save a lot of wasted time and/or material.

Just a thought.

 

[kvom]

When I built Bogstandard's paddleduck engine, which is in metric, I just multiplied all the mm dimensions by .059 (was building at 1.5x). Usually I would just write the inch dimensions on the plans, or do the calculation directly at the mill.

The only non-conversion needed is deciding on fastener sizes. These are typically not that critical, so the nearest size or the most convenient works for me.

 

[GWRdriver]

[snip].03937 is APPROXIMATE.

Yes, the full multiplier is .0393700787401575. Perhaps it would a good idea, in the interest of absolute correctness, if all of us carried our drawing dimensions to 16 decimal places, otherwise disastrous dimensional consequences could possibly occur.

 

[Woodster]

Yes, the full multiplier is .0393700787401575. Perhaps it would a good idea, in the interest of absolute correctness, if all of us carried our drawing dimensions to 16 decimal places, otherwise disastrous dimensional consequences could possibly occur.

Reminds me of a story i read years back where the powers that be decided that for all engineering works, to make the maths of it easier, Pi should be 4!!!
I was always taught that precision engineering is precise therefore you should always aim to be as accurate as your equipment allows.

 

[Maryak]

The easy way to convert imperial to metric is to use 24mm = 1in.

This gives a slightly smaller model in metric than the imperial original but most standard imperial sizes convert to standard metric sizes e.g. 1/4" = 6mm.

Try it and you may be pleasantly surprised at the result.

Best Regards
Bob

 

[GKNIPP]

Thanks for all of your input. I am simply taking the metric dimension, in miilimeters, and multiplying it by .03937 which gets me 4+ place decimals which I feel are more than satisfactory for the task at hand. Yes, i am changing material thickness, shaft diameters and screw threads to nominal imperial sizes and adjusting others where needed.

Basically, I am redrawing sections of the plan, longhand, so I can insure center distances and the such.

Greg

 

[nemt]

Beiing brought up with the metric system,
I convert most drawings from inch to mm by taking 32 mm for the inch. Model becom about 25% bigger. Since I have rather sturdy machines, it is no problem and it works for me.
Just busy building Hit and Mis from David Kersel. First I draw all parts in Solidworks and use the drawings in the workshop.
Nemt

 

[mygrizzly1022]

Hi

There is a unit converter at this link. Which you might find helpful, also included is a scientific calculator. This link was suggested by another poster and I find it extremely helpful. See below

http://www.eeweb.com/toolbox/calculator

Regards .....bert

 

[blighty]

get yours self a calculator.......

take any metric number on your drawing and divide by 25.4 you will then get the decimal equivalent in inches. round up or down to 3 decimal places.

e.g 12.5mm / 25.4 = 0.492125984251 bla bla bla make your bit 0.492"

don't worry about the 0.0001 your part will just be 2.5 microns to small.

 

[Mbusha]

A word of caution!! To divide by 25.4 is EXACT, to multiply by .03937 is APPROXIMATE.

Thank you! Any conversion other than 25.4 is nonsense. I am new to this forum and home model machining but metrology is my life.

Don't get me started. :rant::rant::rant::rant::rant::rant:

 

[littlelocos]

Hello,
Another method I've seen (in one of Kozo Hiraoka's books on logging locomotive construction) was to use 25.6 instead of 25.4. The idea here is that fractional-inch stock can be more easily substituted. 8mm goes to 0.3125" or 5/16". 100mm yields 3.90625 or 3-29/32, odd, but familiar to those of us using the IPS system.

The other way around 1/25.6 = 0.0390625 exactly, so a 2" dimension becomes 51.20mm, etc.

Enjoy,
Todd.

p.s. Mbusha is right..... Use 25.4 ALWAYS if your intent is to make a direct conversion. The inch was redefined as 25.4mm a long time ago. Some history can be found here: http://en.wikipedia.org/wiki/Inch

 

[GKNIPP]

I am not sure what you guys mean, but if I multiply the metric dimension by the .03937 or divide by the 25.4 I get the same answer out 5 or 6 places ?? Not sure why one hold more merit than the other, especially in context I am using them, model engines. Either way I would simply round the answer below to 1.417, however, if the 4th place decimal was greater than 5 I would go to 1.418. Rarely have I needed to utilize or seen 4 place dimensions even used on model engines, much less going out 5, or more. Now in other much more critical applications maybe.

36mm X .03937= 1.41732

36mm / 25.4 = 1.41732

Greg

 

[Mbusha]

Greg, I was going to reply, but I split microns for a living. So I guess this is not the place. Carry on and I'll MYOB and make another Martini.

 

[littlelocos]

Greg, I was going to reply, but I split microns for a living. So I guess this is not the place. Carry on and I'll MYOB and make another Martini.

Ditto, I work for NIST (and am off duty).
www.nist.gov
Carry on,
Todd.

 

[GKNIPP]

I appreciate your input and value it. I apologize if I came across incorrectly but I didn't intend to. I can understand where you are coming from especially when you work with such tiny increments. I have the same problem at times being a toolmaker as I work in tolerances of +/- .0002 very frequently so when I do get a dimension at +/- .005 I feel like I have a mile to work with. I find it hard, at times, to adjust myself. BTW, have a martini for me as well!!!!! LOL!!!!!

Greg